The present invention relates to a portable telephone set and, more particularly, to a portable telephone set having a ringing circuit and to be used in a portable mode and mobile mode.
There has been known a portable telephone set which has two use-modes, i.e., a portable mode in which the telephone set is used by a user while bringing it with him and a mobile mode in which it is mounted on a vehicle. In the portable mode, a compact and light-weight, built-in loudspeaker is used. In the mobile mode, a relatively large external loudspeaker is used because the loudspeaker outputs not only a ringing tone but also a voice signal and thus the voice quality therefrom plays an important role.
In general, in the above-mentioned ringing circuit, the built-in, i.e., internal loudspeaker is compact and has narrow frequency characteristics while the external loudspeaker has relatively wide frequency characteristics. In the conventional ringing circuit, however, a ringing signal having a single frequency spectrum is used. Thus, when the frequency characteristics of the internal and external loudspeakers are different, there may be a problem that the energy of a ringing signal for driving the loudspeaker cannot be used effectively. More definitely, when the ringing signal spectrum is broad and the internal loudspeaker is used, a portion of the frequency spectrum of the ringing signal which is not used increases and the power consumption of a ringing signal generating circuit is undesirably increased. On the other hand, when the ringing signal frequency spectrum is narrowed to adjust it to the internal loudspeaker, it is impossible to supply enough power to the external loudspeaker, so that an effective ringing sound cannot be generated.
Furthermore, the switching between the internal and external loudspeakers is manually carried out. This manual switching is a troublesome operation for the user.